Peak bone mass is determined mainly by genetic-ethnic factors, but environmental factors such as calcium intake and physical activity during childhood and adolescence could play a role. We have measured the bone mineral density (BMD) of 151 healthy children and adolescents, ages 7–153 years. Density was measured by dual X-ray absorptiometry (DXA) at two sites (lumbar verterbrae L1–L4 and the upper femur), and the data were analyzed in terms of the height, weight, sexual maturation, spontaneous calcium intake, and physical activity. Of the children, 57–71% had calcium intakes below 1000 mg/day. BMD increased with pubertal maturation from 0.68 ± 0.08 to 0.92 ± 0.09 g/cm2 (vertebral bone density, VBD) and from 0.87 ± 0.10 to 1.03 ± 0.09 g/cm2 (femoral bone density, FBD) between Tanner stage 1 and 5. Multiple regression analysis showed that body weight and Tanner stage were main determinants of bone density when expressed as g/cm2. The weekly duration of sports activity also influenced both the vertebral (p < 0.001) and femoral (p = 0.01) sites, especially in girls and during puberty. Dietary calcium appeared to be another independent determinant of BMD, especially before puberty, at the vertebral (p = 0.02) site. Most important, dietary calcium was found to be the main determinant of vertebral mineral density, when expressed as z score, in both sexes. Moreover, 93% of the 28 children with low vertebral z score values (below –1) and 84% of the 31 children with low femoral z score values (below –1) had dietary calcium intakes below 1000 mg/day.